1. Field of the Invention
The present invention relates to a variable displacement hydraulic piston pump used for driving a hydraulic motor that in turn drives a refrigerant compressor incorporated in an air-conditioning system of an automobile, and for hydraulically actuating diverse hydraulic devices mounted on special-purpose vehicles including industrial cars such as dump trucks, garbage trucks, and sanitation vehicles. More particularly, the present invention relates to a variable displacement hydraulic piston pump accommodating a self-guard means capable of preventing the input shaft of the pump from being compulsorily rotated under an over-load to thereby protect the pump against mechanical breakage of the internal elements of the pump.
2. Description of the Related Art
Axial piston pumps (it will be referred to as simply a pump hereinbelow) have been used for various industrial machines and industrial vehicles.
FIG. 3 illustrates a conventional variable displacement hydraulic pump provided with a means for adjustably changing an angle of inclination of a swash plate, which causes a reciprocation of axial pistons. The pump is provided with a hollow housing 1, an end covering 2 closing an end of the housing 1, and a crank chamber 3 defined in the closed housing 1. A drive shaft 4 provided to extend through the crank chamber 3 is rotatably supported by bearings 5 seated in the housing 1 and the end covering 2. A cylinder block 6 is mounted on the drive shaft 4 so as to rotate together with the drive shaft 4 in the crank chamber 3. The cylinder block 6 is provided with a plurality of cylinder bores 7 arranged around and in parallel with the rotating axis of the drive shaft 4, and the respective cylinder bores 7 slidably receive reciprocatory pistons 10 therein, which are engaged with a swash plate 9 via shoes 8.
A valve plate 11 is arranged between the open end of the housing 1 and the end covering 2, and is fixed to the inner face of the end covering 2 to seal the respective open-ended cylinder bores 7. The valve plate 11 is provided with a suction port 12a and a discharge port 12b formed as an arcuate-shape through-bore, respectively. Namely, the suction and discharge ports 12a and 12b are circularly elongated so that the respective ports can be in communication with each of the cylinder bores 7 via an opening end 7a for a while during the rotation of the cylinder block 6. The suction and discharge ports 12a and 12b are also in constant communication with suction and discharge bores 13a and 13b, respectively, formed in the end covering 2.
When the cylinder block 6 is rotated together with the drive shaft 4, the respective pistons 10 engaged with the swash plate 9 are reciprocated in the respective cylinder bores 7 to alternately cause an increase and a decrease in the closed volume of the respective cylinder bores 7. Thus, when the closed volume of each cylinder bore 7 is increased, the cylinder bore 7 is in communication with the suction port 12a so as to pump in the operating oil. When the closed volume of each cylinder bore 7 is decreased, the cylinder bore 7 is in communication with the discharge port 12b so as to discharge the operating oil.
The swash plate 9 is pivotally supported by trunnion shafts (not shown in FIG. 3), and is constantly and resiliently urged toward a large inclination-angle position where the swash plate 9 has a large angle of inclination relative to a plane perpendicular to the rotating axis of the drive shaft 4. Namely, a control spring 14 is provided for applying a constant pressing force to the swash plate 9 at a position thereof distant from the pivoting axis thereof.
The swash plate 9 is also engaged with a linearly movable control cylinder 15 at a position thereof diametrically opposed to the above-mentioned position. Thus, when the control cylinder 15 is hydraulically moved forward and back by a pressurized oil supplied from the discharge bore 13b of the pump via a control circuit including an opening and closing valve 16, the swash plate 9 is pivoted about the pivoting axis thereof to increase or decrease the angle of inclination thereof with regard to the plane perpendicular to the axis of the drive shaft 4 against the constant pressing force of the control spring 14. Accordingly, in response to a change in the angle of inclination of the swash plate 9, the theoretical displacement of the pump per revolution of the cylinder block is adjustably changed.
Nevertheless, the above-mentioned variable displacement pump encounters a defect in that since the control spring 14 is arranged so as to constantly urge the swash plate 9 to the large inclination-angle position thereof, when the operation of the pump is stopped, the swash plate 9 is always urged toward the largest inclination-angle position thereof by the spring 14. Namely, when the pump is stopped, the pressure level of the discharge oil of the pump is lowered due to leakage of pressurized oil through a clearance between the respective pistons 10 and the cylinder bores 7 of the cylinder block 6 as well as leakage of pressurized oil from the control cylinder 15 or from a return orifice of the control circuit of the control cylinder 15. Thus, the pressure of the pressurized oil supplied to the control cylinder 15 is insufficient for moving the swash plate 9 toward a small inclination-angle position thereof by overcoming the spring force of the control spring 14. Consequently, when the operation of the pump is started with the swash plate 9 urged toward the largest inclination-angle position, it is required to apply a large starting torque to the pump to start rotation of the drive shaft 4.
Further, since the displacement of the pump is controlled by changing the angle of inclination of the swash plate 9 by using the control cylinder 15 operated by a pressurized oil supplied from the pump per se, it is impossible to bring the angle of inclination of the swash plate 9 to a substantially zero position, because when the swash plate 9 is moved toward a zero inclination-angle position thereof by the control cylinder 15, the pressure level of the discharge oil of the pump is lowered, and accordingly the control cylinder 15 cannot exert a pressing force sufficient for maintaining the zero inclination-angle position of the swash plate 9 against the spring force of the control spring 14. As a result, the pump is unable to perform a continuous small displacement operation. Therefore, it is necessary to provide an appropriate clutch mechanism to disconnect the pump from a drive source such as an automobile engine when no load is applied to the pump.
In order to eliminate the above-mentioned defects of the pump illustrated in FIG. 3, the pending U.S. patent application Ser. No. 07/848,017, now issued as U.S. Pat. No. 5,207,751, corresponding to the pending Japanese Patent Application No. 3-45148 filed by the Applicant which is the same as the assignee company of the present application, discloses a different variable displacement piston pump. The pump is provided with a swash plate, a control spring capable of constantly urging the swash plate toward a small inclination-angle position, a control cylinder capable of pivotally moving the swash plate toward a large inclination-angle position against the spring force of the control spring, and an opening and closing valve arranged in an oil circuit for introducing pressurized oil into the control cylinder.
With the above-mentioned pump, since the swash plate is always urged toward the small inclination-angle position thereof by the control spring, when the operation of the pump is started, it is possible to gradually increase the displacement of the pump from the smallest displacement position close to a zero displacement position by controlling the operation of the control cylinder via the opening and closing valve. Namely, when the swash plate is eventually moved to the largest inclination-angle position against the spring force of the control spring, an ordinary operation of the pump at the largest displacement thereof is obtained. Thus, when no load is applied to the pump, the pump is able to maintain the smallest displacement operation close to a zero displacement operation. Namely, it is not necessary to provide any clutch mechanism between the pump and a pump drive source under no load conditions when the pump is incorporated in a hydraulic operation system for operating hydraulic devices of industrial vehicles. Nevertheless, when a clutch mechanism to disconnect the pump of the hydraulic operation system from the pump drive source is omitted, an unfavorable problem occurs in that, when powdery abraded material or other foreign materials are contained in the discharge oil circuit thereby causing plugging or clogging of the circuit or when a seizure occurs between the pistons and the cylinder bores, between the end face of the rotating cylinder block and the valve plate, and between the swash plate and the shoes due to a lack of lubrication, the drive shaft is driven compulsorily by the pump drive source, i.e., a vehicle engine under an abnormally large load (i.e., an excessive load) applied to the drive shaft.